1. Field of the Invention
The present invention relates to a method and apparatus for producing oxide-series single crystals.
2. Related Art Statement
According to conventional TSSG method, when growing a KLN (potassium lithium niobate K.sub.3 Li.sub.2-2x Nb.sub.5+x O.sub.15+x, hereinafter referred to as KLN) oxide or the like oxide having a composition of solid solution, single crystals of homogeneous optical properties were difficult to produce for the sake of subtle variation of composition of the growing single crystal depending on raw material composition and growing condition. Recently, as a method of growing oxide-series single crystals, a so-called ".mu. pulling down method" for growing single crystal fibers has attracted attentions. Details of growing single crystal fibers by the .mu. pulling down method were described in "DENSOUKEN News", September, 522!, pp 4-8, July, 1993.
Growing of a KLN single crystal from a melt of niobium oxide, potassium carbonate and lithium carbonate and a theoretical study of a relation between the grown KLN single crystal composition and second harmonic phase-matching wavelength referring to change of lattice constants, were reported in "HIKARI GIJYUTSU KONTAKUTO (Optical and electro-optical engineering contact)", 33, 7!, (1995). In the report, phase matching was attained at a wavelength of 830 nm when the single crystal has 51 mol % of niobium in the composition, while phase matching was obtained at a wavelength of 880 nm when the single crystal has 53 mol % of niobium in the composition. In this way, phase matching wavelength was shown to change depending upon the composition of the single crystal. More concretely, with the increase of niobium content by 1 mol %, SHG (second harmonic generation) phase matching wavelength was shifted to a longer wavelength side by 25 nm. Also, it was shown that, in order to control the SHG phase matching wavelength within a range of .+-.1.0 nm, the composition of KLN single crystal has to be controlled within a range of .+-.0.04 mol %.
In addition, when growing a KLN single crystal at a growing rate of about 6 mm/hr in the .mu. pulling down method, a tendency was observed that a phase matching wavelength of the single crystal was shifted to a shorter wavelength side with the progress of growing as reported in "the 39th JINKOU KESSYOU TOURONKAI", Oct. 17, 1994. This is considered due to the followings. As seen from the phase diagram of KLN, single crystal of a higher niobium content than that of the melt in the crucible is continuously grown, so that niobium content in the melt is gradually decreased. Accompanying with the decrease of niobium content in the melt, niobium content in the grown single crystal is decreased to shift the phase matching wavelength to a shorter wavelength side. Therefore, in the .mu. pulling down metbod, continuous growing of the single crystal having target optical properties, particularly a phase matching wavelength, was difficult to achieve.
Thus, when producing a material for elements for a second harmonic generation or a solid laser, a small variation of composition of the single crystal results in a noticeable variation of property of the produced single crystal to produce unacceptable products. Therefore, variation of composition of the single crystal has to be prevented as far as possible, when continuously pulling down the single crystal.